The type of electrode which is in almost universal use in high pressure metal vapor lamps comprises a rod or shank around which is wound a tungsten coil structure. A common design is a two-layer coil wherein the inner layer has spaced turns and the outer layer is close wound and screwed over the first. The interstices between turns are filled with emissive material which is retained in place by the outer layer. Emissive materials commonly used are oxides of low work function materials such as thorium oxide or mixtures of alkaline earth oxides including barium oxide. The shank projects through the coil and forms a tip to which the arc attaches with formation of a hot spot.
In such electrodes the emissive material or activator reaches the tip by diffusion over the surface in a process which is strongly temperature dependent. With a high tip temperature, electron emission is large and the cathode fall is low but the evaporation rate of emission material is large. Reducing the tip temperature reduces the evaporation rate but electron emission is decreased, cathode fall increases and sputtering by ion bombardment may take place. The common electrode design problem is to arrange the dispensing of the activator so that it balances the rate of loss. Since this cannot be done exactly, the compromise generally adopted is to design the electrode for an excess rate of activator supplied to the tip and to provide an ample reservoir of emissive material in the coil structure. Accordingly, the electrodes of high intensity discharge lamps have tended to be relatively large massive wound tungsten coil structures which are difficult to locate accurately in the ends of the fused silica envelopes into which they are pinch-sealed.
It is known that in metal halide lamps having a fill including thermally decomposing metal halides such as thorium iodide ThI.sub.4, pyrolytic decomposition of the thorium iodide followed by condensation of thorium metal on the electrode surface yields a surface which emits electrons efficiently. The thorium layer shields the tungsten from erosion. As for the thorium, an iodine transport cycle continually replenishes the quantity of thorium on the electrode tip. An efficient electrode activation system is thus available but up to the present it has been used with the relatively massive wound tungsten electrode structures developed for the prior art electron emissive materials. The object of the invention is to provide improved and more efficient electrode structures taking greater advantage of the characteristics of pyrolitically decomposing metal halides for electrode activation.